PROJECT SUMMARY/ABSTRACT The number of Americans at risk for hepatocellular carcinoma (HCC) is increasing due to the growing obesity epidemic and its association with fatty liver disease, steatohepatitis, and cirrhosis. No medications have been shown to prevent HCC in high-risk patients with chronic liver disease. The long-term goal is to define mechanisms of hepatocarcinogenesis, providing a foundation for improved preventives, diagnostic tools, and treatments for HCC. The overall objective of this proposal is to identify cellular and molecular mechanisms by which serotonin promotes and antidepressants inhibit liver growth and tumor formation. The central hypothesis of this proposal is that 1) serotonin promotes liver growth and tumorigenesis by stimulating the serotonin receptor HTR2A on hepatocytes; and 2) serotonergic antidepressants inhibit liver growth and tumorigenesis by antagonism of HTR2A. The following specific aims are proposed to test this hypothesis: 1) Determine the cellular and molecular role of HTR2A in promoting liver tumor cell viability and liver growth; 2) Determine if amitriptyline suppresses liver growth and tumor formation via HTR2A; 3) Identify downstream genes that mediate amitriptyline?s effects on ?-catenin-driven liver growth and tumorigenesis and determine whether amitriptyline?s effects required activated ?-catenin. In Aim 1, the hypothesis that HTR2A mediates serotonin?s growth-promoting effects on HCC cells will be tested by measuring the effect of HTR2A knockdown on serotonin responsiveness of human liver cancer cell lines. In parallel, HTR2A orthologs will be knocked down in a zebrafish HCC model to determine the receptor and cell type(s) that mediate serotonin?s growth-promoting effects on hepatocytes in vivo. In Aim 2, the hypothesis that amitriptyline suppresses liver growth and tumor formation via inhibition of HTR2A will be tested using complementary approaches in human liver cancer cell lines and zebrafish as in Aim 1. In Aim 3, a prioritized list of genes, downregulated by amitriptyline treatment in mouse HCC, will be characterized for effects on liver growth and tumorigenesis and response to amitriptyline using zebrafish and mouse HCC models. The hypothesis that amitriptyline?s actions require activated ?-catenin will be tested. We expect the proposed studies to define how serotonin signaling and serotonergic antidepressants modulate liver growth and tumorigenesis. This research is significant because it will form the basis for identifying drugs to prevent liver cancer in high-risk patients, which could save thousands of lives each year.